Along with dark matter and dark energy, researchers looking at the CMB …

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Only five percent of the universe is now thought to consist of normal matter, with 23 percent of it being dark matter and the remaining 72 percent dark energy. Astrophysicists and astronomers have a large amount of data indicating the existence of dark matter, although there is still an open debate about what it is made of. Dark energy, on the other hand, has only one real piece of corroborating evidence: the accelerating expansion of the universe (stay tuned to Nobel Intent for a completely different take on this phenomena in the near future). Now, to add to the dark, researchers examining the cosmic microwave background radiation (CMB) have identified what they term a "dark flow."

A quartet of researchers measured fluctuations in the CMB that result from the scattering of microwave photons by energetic X-ray emissions from galactic clusters, and discovered a coherent flow of matter across the universe. Dubbed "dark flow" by the team, it cannot easily be explained by the distribution of matter in the visible universe. The team postulates that this motion may be the effect of matter residing outside the CMB—something beyond our ability to directly detect. Their paper is set to be published in an upcoming edition of the Astrophysical Journal Letters, but a free pre-print can be found on the arXiv archives.

The white dots are the 700 clusters worth of hot gas,
while the pink ellipse is where they are all headed
Image Credit: NASA/WMAP/A. Kashlinsky et al.

The data used in the paper came from the three year Wilkinson Microwave Anisotropy Probe (WMAP) dataset. Using the WMAP data, the researchers extracted the wavelength of scattered photons from individual galactic clusters. Since the clusters' motion does not exactly follow the expansion of space-time, these scattering measurements allow researchers to compute the individual motions of each cluster. This is apparent in a very small change in the CMB temperature in the direction the clusters are flowing, a phenomena known as the kinematic Sunyaev-Zel'dovich effect. This technique has a drawback, in that the measurements of this effect for a single cluster have a large statistical errors; to overcome this, the researchers took measurements from over 700 distinct clusters.

The velocity of these clusters was computed to be around 2 million miles per hour. Once the part of the movement that is caused by the expansion of the universe was removed, the researchers found a coherent direction to the remaining flow—matter seems to stream towards a region of space between the constellations Centaurus and Vela.

One of the leading theories used to explain the properties observed in the CMB is the ΛCDM concordance model of the universe. In this model, velocities should be decreasing with the distance from the center of the universe and should show no particular orientation. The new observations fly in the face of a model with these properties.

Most cosmological models suggest that, shortly after the Big Bang occurred, the universe underwent a period of inflation—a time when it grew in size at unimaginable rates. In certain inflationary models, the observable universe is described as a "homogeneous inflated region embedded in an inhomogeneous space-time." That may mean that there are still regions outside the observable universe that haven't inflated or are undergoing an inflation with different properties from our region.

The researchers point out that, at very large scales, these remnants of the pre-inflationary universe may actually exert force on what we can see, resulting in a tilt in the CMB. According to lead author of the paper Alexander Kashlinsky, "this measurement may give us a way to explore the state of the cosmos before inflation occurred."

To continue this line of research, the team wants to reduce the error bars by gathering more data about how the hot gas is distributed with in these clusters. The team is currently assembling a larger catalog of these clusters by using the full five year data set from the WMAP.

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Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm